Sorry I was offline for a bit.
Lipomas are fatty tumors and some can change and become malignant at which point they are called infiltrative lipomas. Most though are removed because they are disfiguring to the dogs looks or are bothering the dog.
You can read about lipomas here
http://www.veterinarypartner.com/Content.plx?P=A&C=189&A=2551&S=0
While you should have any new ones checked by your vet, if the two he has removed check out as not malignant you might consider just doing fine needle aspirate biopsies on any new ones and only remove them if they change or become bothersome.
Hope this helps you!
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